The present invention relates to a method, a computer program product, and an apparatus for designating region of interest of a subject, such as an internal organ.
Conventionally, medical image information having three or more dimensions (volume data) is generated and visualized by medical diagnostic imaging devices, such as X-ray diagnostic devices, X-ray CT devices, magnetic resonance imaging devices (MRI devices) and the like. Visualized information is used to diagnose and treat illness. It is difficult to effectively visualize internal body areas since volume data is configured by data of tightly packed elements (voxel) having three or more dimensions. In general, there are two well known methods to visualize volume data. One is to extract a region of voxel data subjected to visualization, that is, a region of interest (ROI). Then, voxel data included in ROI can be rendered by raycast or any other volume rendering method. The other is to cut out two-dimensional plane in volume data and render voxels on the plane.
Although the region of interest can be set by region extraction, there are many region extraction methods considered suitable for subjects such as internal organs and substances. One example of region extraction is described below. First, the volume data configured by voxels having variable density values are binarized by a predetermined threshold value to generate binary voxel data. Thereafter, light rays are extended perpendicular to the binary voxel data, the lengths of the rays to voxels (region of interest voxel) having a value of “1” are determined, and a depth image is generated from the lengths of the rays to each pixel. Then, coordinates of the surface of the display subject are reversely calculated from the depth image, a surface normal line is determined and shadowed based on the voxel values of certain voxel data and the voxel values of adjacent voxel data to generate a surface display image (for example, refer to U.S. Patent Publication No. 5,793,375).
Multi planar reconstruction (MPR) is a two-dimensional extraction method. MPR is a method for extracting and displaying arbitrary cross-sectional planes of voxel data, for example, a slice image of a cross-section that differs from a plurality of slice images (CT images) imaged on a horizontal cross-section.
Since MPR represents an arbitrary cross-sectional plane, however, it is difficult to observe a subject that is on a curved surface and not on a flat surface, as in the case, for example, of blood vessels on the surface of internal organs. A region of interest between two flat planes is cut out from the volume data, and this region of interest is subjected to slab MIP, in which a MIP (max intensity projection) process is performed. In MIP, an optical path is set so as to pass through voxel data. Then, the maximum data (pixel value of each pixel of medical image information) on the optical path are displayed as a MIP image. Since the MIP process is limited to a region of interest between two planes in the voxel data, from the viewpoint that a thick plane region can be expressed, slab MIP is an intermediate method with respect to visualization methods the extract a two-dimensional plane and visualization methods using a region of interest. Although slab MIP allows easy observation of a subject that zigzags through a region between two flat planes, such as a blood vessel, the observation range is restricted to being within a predetermined observable region.
Furthermore, a curved cross-section can be visualized by CPR (curved multi planar reconstruction). CPR is a method for extracting and displaying a curved section of voxel data, for example, when observing a blood vessel, a curved section can be extracted so as to enable a blood vessel to always be displayed in cross-section. Freedom of selecting the curved surface is limited since the curved surface generated by CPR is displayed as a collection of parallel lines on a single curve. Therefore, although it is possible, for example, to generate a curved section including a single blood vessel, it is not possible to express a curved surface that includes a plurality of curves, as in the case of a branching blood vessel and the like. Furthermore, even when one blood vessel is displayed using CPR, a surface along the blood vessel must be specified by an operation performed by the user. This increases the labor involved in display and observation.